If you use lightdm-gtk-greeter instead of the default unity-greeter you can specify the login window position since version 1.7.0:
Edit your greeter conf file (/etc/lightdm/lightdm-gtk-greeter.conf) and set the position:

# 960px from the left and 25% from the top
position=960 25%
# or 25% from the right and 400px from the bottom
position=-25% -400